1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system which is capable of a close photography.
2. Description of the Prior Art
Conventionally, a zoom lens system has widely been used in a 35 mm camera. However, in the conventional zoom lens system, the shortest object distance is long and the highest magnification is low compared to a fixed focal length lens system having approximately the same focal length. To compensate for this defect, a method (so called macro method) is employed to reduce the shortest object distance by moving along the optical axis a lens unit other than the lens unit moved for focusing.
However, according to the conventional macro method, since two kinds of focusing operations: a normal focusing operation and a focusing operation for a very close object are provided, the structure of the lens barrel is necessarily very complicated. In addition, since the shorter the object distance is, the more remarkable the generation of aberration is, sufficient image formation performance cannot be obtained in close photography.
Further, it is generally known that the first lens unit is moved out in focusing of the conventional zoom lens system. Although this method is simple and easy, sufficient image formation performance cannot be obtained in close photography since the image plane inclines.
Conventionally, in many high-magnification zoom lens systems including a long focal length, focusing is performed according to a front lens moving out method where the most object side lens unit (front lens) is moved out.
However, this method is not suitable for a zoom lens system of automatic focusing type since the front lens of the high-magnification zoom lens is heavy and the heavy lens unit must be moved. Since the moving out amount of the front lens is necessarily large, the size of the lens barrel increases. Further, since the optical performance deteriorates in close photography, it is impossible to secure sufficiently excellent performance up to a sufficiently high enlargement magnification.